Researchers now estimate that there are more than 360,000 western lowland gorillas in the wild, approximately one third higher than earlier figures.
Photograph: Zanne Labuschagne/WCS

There are far more gorillas left in the world than previously thought, according to a landmark new survey, with numbers as much as double earlier estimates.

However, their populations are continuing to fall fast, down 20% in just eight years, leaving them critically endangered. Furthermore, 80% of the remaining gorilla troops do not live in protected areas, leaving them vulnerable to the threats the researchers summarise as “guns, germs and [felled] trees”.

The decade-long survey in western equatorial Africa involved almost 9,000km of foot patrols and used the nests that gorillas make every night to assess the population. The scientists covered the entire range of the western lowland gorilla, which accounts for 99% of all living gorillas, now thought to number around 360,000 animals.

Gorillas are vital for the health of the entire forest, the scientists said, because they spread large seeds and their loss is disastrous in the long term. The larger-than-expected population gives hope for gorilla survival, they said, if poaching and forest destruction for palm oil can be halted.

“The population could be double,” said Prof Fiona Maisels, from the Wildlife Conservation Society and the one of 50-strong international experts who conducted the survey. “But that is not the big story. Just because there are rather a lot of them does not mean they are not very, very vulnerable.” Gorillas breed very slowly, with females taking 11-12 years to reach maturity and only giving birth every four years. “It takes a very long time to build populations back up,” Maisels said.

The survey, published in the journal Science Advances, covered about a quarter of the gorilla’s total range - an area the size of France - which is naturally bounded by the Atlantic Ocean and the Congo and Ubangi rivers. They then used the nest counts and data on important factors including the presence of people, roads and intact forest to produce a sophisticated statistical model.

This allowed them to fill in the gaps between the surveyed areas, unlike previous research. The resulting total was 361,900 gorillas, far higher than the earlier estimates of 150,000 to 250,000. The researchers also assessed the population of the central chimpanzee sub-species, which lives in the same range and represents about a third of all chimps. Their population was 10-80% higher than thought.

The western lowland gorilla is by far the most populous subspecies. Photograph: Forrest Hogg

It is illegal to kill or capture any great ape in all the nations where they live, but poaching to supply bushmeat to fast-growing cities is rife. Outbreaks of Ebola virus have also decimated populations in recent decades, with “catastrophic” impact on populations, the scientists said. “If Ebola wipes through an area, they all go – 90% of them in six months,” said Maisels.

The most important single factor in determining the survival of gorillas was the presence of park guards to deter poachers, the analysis found. Dominant males are easy to shoot because they attempt to stand up to hunters, trying to protect their group. The males replacing them often kill the babies they have not fathered, compounding the losses.

In contrast, chimpanzees, which spend much more time in trees, usually flee when hunters approach. “They are not as heroic, if you like,” said Maisels. “Chimps are cleverer and sneak away very quietly.” Local cultural taboos against eating chimp meat also helped protect them in a few locations. The chimp population did not show a significant decline in the study period from 2008 to 2013.

“The advice we would like to offer is keep up the anti-poaching effort,” said Maisels. “That gets rid of the immediate worry. But the future worry is the removal of forest for agricultural development. If a big area of forest is turned into an oil palm or rubber plantation, it is not habitat for anything anymore, except weeds.”

Only 20% of the gorilla’s range is in protected areas and the researchers said the vast majority of unprotected forests are being opened up to logging and deforestation: “It is vital that we step up our efforts to conserve great apes.”

The study produced a total population estimate for the central chimpanzee of nearly 130,000 individuals. Photograph: Emma Stokes/WCS

The situation varies widely by country, said Maisel, with Gabon having good national policies while the war-torn Central African Republic is struggling to govern at all. Congo is a hotspot for gorillas, she said, and could play a vital role in conservation.

Liz Williamson, great apes coordinator for the International Union for Conservation of Nature’s red list, said a combination of responsible development and a network of well-managed parks would provide a winning formula: “Our study has revealed that it is not too late to secure a future for gorillas.”

The western lowland gorilla is by far the most populous subspecies. There are about 3,600 eastern lowland or Grauer’s gorillas – a fall of 75% in last 20 years - 800 mountain gorillas and just 200-300 Cross River gorillas.